Vodacom put me under a contract without my approval

I Snowwhite Kubayi (aged 69) took a promotion at vodacom for R55 and the original price was R65 and the promotion gave me R65 worth of airtime. I was told by the vodacom salesperson that I could cancel the deal at anytime. Recently vodacom has told me I need to pay R70 per month for the airtime, I tried to cancel the service and I was told by vodacom that the cancelation fee is R800 which I cannot afford as I am currently a pensioner and use most of my earnings for medication. They have put me under a contract without my approval.

Please help me out. Thank you.